The invention relates to drilling fluid additives which suppress clay swelling within a subterranean well during the drilling process. The invention is particularly directed to hydration inhibiting additives for drilling fluids comprising hydroxyalkyl quaternary ammonium compounds which are compatible with anionic polymers typically found in or added to drilling fluids and are environmentally acceptable.
In rotary drilling of subterranean wells numerous functions and characteristics are expected of a drilling fluid. A drilling fluid should circulate throughout the well and carry cuttings from beneath the bit, transport the cuttings up the annulus, and allow their separation at the surface. At the same time, the drilling fluid is expected to cool and clean the drill bit, reduce friction between the drill string and the sides of the hole, and maintain stability in the borehole's uncased sections. The drilling fluid should also form a thin, low-permeability filter cake that seals openings in formations penetrated by the bit and act to reduce the unwanted influx of formation fluids from permeable rocks.
Drilling fluids are typically classified according to their base material or primary continuous phase. In oil-base fluids, solid particles are suspended in oil, and water or brine may be emulsified with the oil. The oil is typically the continuous phase. In water-base fluids, solid particles are suspended in water or brine, and oil may be emulsified in the water. Water is the continuous phase. Oil-base fluids are generally more effective in stabilizing water-sensitive shales than water-base fluids. However, environmental concerns have limited the use of oil-base drilling fluids. Accordingly, oil drilling companies have increasingly focused on water-base fluids.
Three types of solids are usually found in water-base drilling fluids: (1) clays and organic colloids added to provide necessary viscosity and filtration properties, (2) heavy minerals whose function is to increase the drilling fluid's density, and (3) formation solids that become dispersed in the drilling fluid during the drilling operation.
The formation solids that become dispersed in a drilling fluid are typically the cuttings produced by the drill bit's action and the solids produced by borehole instability. Where the formation solids are clay minerals that swell, the presence of such solids in the drilling fluid can greatly increase drilling time and costs. The overall increase in bulk volume accompanying clay swelling impedes removal of cuttings from beneath the drill bit, increases friction between the drill string and the sides of the borehole, and inhibits formation of the thin filter cake that seals formations. Clay swelling can also create other drilling problems such as loss of circulation or pipe sticking that can slow drilling and increase the drilling costs.
In the North Sea and the United States gulf coast areas, drillers commonly encounter argillaceous sediments in which the predominant clay mineral is montmorillonite (commonly called "gumbo shale"). Gumbo shale is notorious for its swelling. Thus, given the frequency in which gumbo shale is encountered in drilling subterranean wells, the development of a substance and method for reducing clay swelling has long been a goal of the oil and gas exploration industry.
The mechanisms of clay swelling are well known. Clay minerals are crystalline in nature. The structure of a clay's crystals determines its properties. Typically, clays have a flaky, mica-type structure. Clay flakes are made up of a number of crystal platelets stacked face-to-face. Each platelet is called a unit layer, and the surfaces of the unit layer are called basal surfaces.
A unit layer is composed of multiple sheets. One sheet is called the octahedral sheet, and is composed of either aluminum or magnesium atoms octahedrally coordinated with the oxygen atoms of hydroxyls. Another sheet is called the tetrahedral sheet. The tetrahedral sheet consists of silicon atoms tetrahedrally coordinated with oxygen atoms.
Sheets within a unit layer link together by sharing oxygen atoms. When this linking occurs between one octahedral and one tetrahedral sheet, one basal surface consists of exposed oxygen atoms while the other basal surface has exposed hydroxyls. It is also quite common for two tetrahedral sheets to bond with one octahedral sheet by sharing oxygen atoms. The resulting structure, known as the Hoffman structure, has an octahedral sheet that is sandwiched between the two tetrahedral sheets. As a result, both basal surfaces in a Hoffman structure are composed of exposed oxygen atoms.
The unit layers stack together face-to-face and are held in place by weak attractive forces. The distance between corresponding planes in adjacent unit layers is called the c-spacing. A clay crystal structure with a unit layer consisting of three sheets typically has a c-spacing of about 9.5.times.10.sup.-7 mm.
In clay mineral crystals, atoms having different valences commonly will be positioned within the sheets of the structure to create a negative potential at the crystal surface. In that case, a cation is adsorbed on the surface. These adsorbed cations are called exchangeable cations because they can trade places with other cations when the clay crystal is in water. In addition, ions can also be adsorbed on the clay crystal edges and exchanged with other ions in the water.
The type of substitutions occurring within the clay crystal structure and the exchangeable cations adsorbed on the crystal surface greatly affect clay swelling, a property of primary importance in the drilling fluid industry. Clay swelling is a phenomenon in which water molecules surround a clay crystal structure and position themselves to increase the structure's c-spacing. Two types of swelling can occur.
Surface hydration is one type of swelling in which water molecules are adsorbed on crystal surfaces. Hydrogen bonding holds a layer of water molecules to the oxygen atoms exposed on the crystal surfaces. Subsequent layers of water molecules then line up to form a quasi-crystalline structure between unit layers which results in an increased c-spacing. All types of clays swell in this manner.
Osmotic swelling is a second type of swelling. Where the concentration of cations between unit layers in a clay mineral is higher than the cation concentration in the surrounding water, water is drawn between the unit layers and the c-spacing is increased. Osmotic swelling results in larger overall volume increases than surface hydration. However, only certain clays, like sodium montmorillonite, swell in this manner.
Exchangeable cations found in clay minerals greatly impact the amount of swelling that takes place. The exchangeable cations compete with water molecules for the available reactive sites in the clay structure. Generally, cations with high valences are more strongly adsorbed than ones with low valences. Thus, clays with low valence exchangeable cations will swell more than clays whose exchangeable cations have high valences. Calcium and sodium cations are the most common exchangeable cations in gumbo shale. As the sodium cation has a low valence, it easily disperses into water, thereby giving gumbo shale its notorious swelling characteristics.
Although a number of compounds are known for their effectiveness in inhibiting reactive shale formations, several factors affect the practicality of using swelling inhibitor additives in drilling fluids. First, the inhibitor must be compatible with the other drilling fluid components. The driller of subterranean wells must be able to control the rheological properties of drilling fluids by using additives such as bentonite, anionic polymers and weighting agents. Thus, drilling fluid additives should also provide desirable results but should not inhibit the desired performance of other additives. However, many swelling inhibitors will react with other drilling fluid components, resulting in severe flocculation or precipitation.
Second, current drilling fluid components must be environmentally acceptable. As drilling operations impact on plant and animal life, drilling fluid additives should have low toxicity levels and should be easy to handle and to use to minimize the dangers of environmental pollution and harm to personnel. Moreover, in the oil and gas industry today, it is desirable that additives work both onshore and offshore and in fresh and salt water environments.
Numerous attempts have been made to improve the shale inhibition of water-base drilling fluids. One method to reduce clay swelling is to use inorganic salts in drilling fluids, such as potassium chloride and calcium chloride. Other methods examined for controlling clay swelling have centered on the use of water soluble polymers in drilling fluids. Since they adsorb on the surfaces of clays when included in drilling fluids, these polymers compete with water molecules for the reactive sites on clays and thus serve to reduce clay swelling. These polymers can be either cationic, anionic, or nonionic. Cationic polymers dissociate into organic cations and inorganic anions, while anionic polymers dissociate into inorganic cations and organic anions. Nonionic polymers do not dissociate. Cationic polymers have proven to be generally more effective shale inhibitors than either anionic or nonionic polymers.
Several cationic polymer systems for water-base fluids have been proposed. One system, a brine-base system, examined two dialkyldimethyl quaternary ammonium salts (dialkyl quats) of the following general formula: ##STR1## wherein x=10 or 16. Although the shorter chain dialkyl quat (x=10) was more effective in inhibiting shale than the longer chain dialkyl quat (x=16), the tests indicated that the ability of the dialkyl quats to inhibit shale appeared to be hindered by their limited solubility in water.
Another attempt examined three trimethylalkyl ammonium chlorides (mono alkyl quats) of the following general formula: ##STR2## wherein x=10, 14 or 16.
The alkyl quat with the shortest chain (x=10) showed the best shale inhibition. However, drilling fluids formulated using the alkyl quat in conjunction with potassium chloride in a drilling fluid formulation generated large amounts of foam. Consequently, the three alkyl quats were judged unsuitable for use in drilling.
Based on the failure of brine-base systems employing potassium chloride and quaternary compounds, alternative cationic polymers were evaluated. Cationic polymers were again used in conjunction with potassium chloride. The brine-base system employed potassium chloride and three additional quaternized polymers having the following general formulas: ##STR3##
A Formula I category polymer exhibited the best shale inhibition. A drilling fluid formula was prepared using conventional viscosifiers, fluid loss additives, the shale inhibitor of formula I and potassium chloride. The cationic polymer was found to be incompatible with the conventional anionic additives, i.e., bentonite, xanthan gum, carboxymethylcellulose (CMC), polyacrylates, etc. A non-ionic viscosifier, hydroxyethylcellulose, and a non-ionic fluid loss agent, pregelatinized starch, were used as substitutes to overcome the incompatibility problem. Further details regarding the brine-base systems described above are reported in Beihoffer et al., "The Development of an Inhibitive Cationic Drilling Fluid for Slim-Hole Coring Applications," SPE-19953 presented at the 1990 SPE/IADC Drilling Conference held in Houston, Feb. 27-Mar. 2, 1990, the subject matter of which is incorporated herein by reference.
Although the described cationic polymers are effective shale inhibitors, the incompatibility of the polymers with common anionic drilling fluid additives is a disadvantage. Moreover, these cationic polymers are toxic. Since environmental concerns are of ever increasing importance, a search for compatible cationic polymers having low toxicity has resulted.
One effort identified two cationic polymers having low toxicity and good shale inhibition when used together. The first polymer is a high molecular weight cationic polyacrylamide. The second polymer is a quaternary polyamine. In the drilling fluid formulation, the high molecular weight polyacrylamide was used for shale encapsulation, and the low molecular weight polyamine was used for swelling suppression. Although the two polymers had low toxicity, they were totally incompatible with anionic polymers in fresh water. Adding salts, such as sodium chloride, to increase the ionic concentration, alleviated the precipitation problem. However, the polymers also caused flocculation of the bentonite component of the drilling fluid. This problem was corrected by adding polyvinyl alcohol to the formulation as a deflocculant. Additional details of the described system, including toxicity tests and additional background on water adsorption and shale inhibition are in Retz, et al., "An Environmentally Acceptable and Field-Practical, Cationic Polymer Mud System," SPE-23064 presented at the Offshore Europe Conference held in Aberdeen, Sep. 3-6, 1991, the subject matter of which is incorporated herein by reference.
Although research has identified cationic polymers which are effective shale inhibitors for use as drilling fluid additives, other cationic polymers with improved compatibility and low toxicity are desired.
A variety of fluids are used during and after drilling operations in subterranean earth formations. A clear distinction is drawn in the drilling fluids art between fluids that are actively used during drilling operations and fluids that are used after drilling operations. One type of fluid used after drilling operations is referred to as a fracturing fluid. Fracturing fluids are materials injected into the producing portion of a well formation in order to "fracture" the formation in which the hydrocarbons are maintained to permit ease of flow and ultimate removal. Such a fracturing fluid is taught by U.S. Pat. No. 5,097,904. Notably, the fracturing fluids typical in the oil well drilling industry do not include any of additives that are typically found in drilling fluids. In particular, drilling fluid characteristics such as toxicity and compatibility with anionic materials are not important to a fracturing fluid. Weight materials are not incorporated into fracturing fluids as they are in drilling fluids.